Microparticle Cloud Research Articles

Diagnostics of the influence of levitating microparticles on the radiofrequency argon plasma

The effect of a levitating cloud of microparticles on the parameters of a radiofrequency (RF) plasma has been studied by means of two experimental techniques. Axial distributions of 1 s excited states of argon were measured by a self-absorption method. A correction of a standard self-absorption method for the extinction of the light by the levitating microparticles is proposed. In addition the electron temperature was estimated using the optical emission spectroscopy. Measurements at the same discharge conditions in a microparticle-free discharge and discharge, containing a cloud of levitating microparticles, revealed the non-local influence of the microparticle cloud on the discharge plasma. The most probable cause of this influence is the disturbance of the ionization balance by the levitating microparticles.

Spectroscopic evaluation of the effect of the microparticles on radiofrequency argon plasma

Axial distributions of 1s excited states of argon were measured in a radiofrequency (RF) discharge by a self-absorption method. Experiments were performed in the PK-3+ chamber, designed for microgravity experiments in complex (dusty) plasmas on board the International Space Station. A correction of a standard self-absorption method for the extinction of the light by the levitating microparticles is proposed. Distributions, measured at the same discharge conditions in a microparticle-free discharge and a discharge containing a cloud of levitating microparticles, revealed the non-local influence of the microparticle cloud on the discharge plasma. The most probable cause of this influence is the disturbance of the ionization balance by the levitating microparticles.

Convective Dust Clouds Driven by Thermal Creep in a Complex Plasma

Steady-state clouds of microparticles were observed, levitating in a low-frequency glow discharge generated in an elongated vertical glass tube. A heated ring was attached to the tube wall outside, so that the particles, exhibiting a global convective motion, were confined vertically in the region above the location of the heater. It is shown that the particle vortices were induced by the convection of neutral gas, and the mechanism responsible for the gas convection was the thermal creep along the inhomogeneously heated tube walls. The phenomenon of thermal creep, which commonly occurs in rarefied gases under the presence of thermal gradients, should generally play a substantial role in experiments with complex plasmas.

Highly Resolved Self-Excited Density Waves in a Complex Plasma

Experimental results on self-excited density waves in a complex plasma are presented. An argon plasma is produced in a capacitively coupled rf discharge at a low power and gas pressure. A cloud of microparticles is subjected to effective gravity in the range of 1-4 g by thermophoresis. The cloud is stretched horizontally (width/height approximately 45 mm/8 mm). The critical pressure for the onset of the waves increases with the temperature gradient. The waves are propagating in the direction of the ion drift. The wave frequency, phase velocity, and wavelength are measured, and particle migrations affected by the waves are analyzed at a time scale of 1 ms/frame and a subpixel space resolution.

Force field inside the void in complex plasmas under microgravity conditions

Observations of complex plasmas under microgravity conditions onboard the International Space Station performed with the Plasma-Kristall experiment-Nefedov facility are reported. A weak instability of the boundary between the central void (region free of microparticles) and the microparticle cloud is observed at low gas pressures. The instability leads to periodic injections of a relatively small number of particles into the void region (by analogy this effect is called the "trampoline effect"). The trajectories of injected particles are analyzed providing information on the force field inside the void. The experimental results are compared with theory which assumes that the most important forces inside the void are the electric and the ion drag forces. Good agreement is found clearly indicating that under conditions investigated the void formation is caused by the ion drag force.

Development of an optical trap for microparticle clouds in dilute gases

Long-duration experiments with clouds of microparticles are planned for the ICAPS facility on board the International Space Station ISS. The scientific objectives of such experiments are widespread and are ranging from the simulation of aerosol behaviour in Earth's atmosphere to the formation of planets in the early solar system. It is, however, even under microgravity conditions, impossible to sustain a cloud of free-floating, microscopic particles for an extended period of time, due to thermal diffusion and due to unavoidable external accelerations. Therefore, a trap for dust clouds is required which prevents the diffusion of the particles, which provides a source of relative velocities between the dust grains and which can also concentrate the dust to higher number densities that are otherwise not achievable. We are planning to use the photophoretic effect for such a particle trap. First short-duration microgravity experiments on the photophoretic motion of microscopic particles show that such an optical particle-cloud trap is feasible. First tests of a two-dimensional trap were performed in the Bremen drop tower.

Decharging of complex plasmas: first kinetic observations.

The first experiment on the decharging of a complex plasma in microgravity conditions was conducted. After switching off the rf power, in the afterglow plasma, ions and electrons rapidly recombine and leave a cloud of charged microparticles. Because of microgravity, the particles remain suspended in the experimental chamber for a sufficiently long time, allowing precise measurements of the rest particle charge. A simple theoretical model for the decharging is proposed which agrees quite well with the experiment results and predicts the rest charge at lower gas pressures.

Observations of structured and long-range transport in a large volume dusty (complex) plasma experiment

DUPLEX—the DUsty PLasma EXperiment at the Naval Research Laboratory—is a large volume dc glow discharge plasma device. The DUPLEX chamber is a transparent polycarbonate cylinder that is 40 cm in radius and 80 cm in height. Argon dc glow discharge plasmas are generated between a grounded cathode and a biased anode. The anode and cathode are separated by 15 to 20 cm. Clouds of 1 μm diameter alumina microparticles are suspended in the plasma—with the highest density directly above the cathode. However, particles have been detected throughout the entire plasma volume. This article reports on previously unidentified, and possibly unknown, phenomena observed in a dusty plasma. Two specific features will be the focus of this article. First, large microparticle clouds—up to 15 cm in diameter in some locations—with highly complex internal structures are identified. Second, long range—20 to 30 cm distances—periodic transport of microparticles from one region of the plasma to another is identified. It is believed that the large size and nonconducting boundary of the DUPLEX chamber facilitates the observation of these phenomena.

Potential profiles obtained from applied dust cloud perturbations

This paper details an experimental investigation of the local potential structure within a cloud of suspended microparticles—a “dusty” or “complex” plasma—using particle image velocimetry (PIV) techniques. Applied perturbations, synchronized to the PIV measurements, are used to force a cloud of suspended microparticles to become unconfined. From the free-streaming motion of the particles during the loss of confinement and subsequent reformation of the dust cloud, an analysis of the potential is performed. Furthermore, a new method of analyzing the potential structure from the motion of free-streaming microparticles in the plasma is presented.

A microparticle plasma

A study is made of a cloud composed of microparticles which are individually either positively or negatively charged. The plasma definitions that the total charge of the cloud is zero and the shielding distance is small compared to the dimensions of the system are taken to be satisfied. As a result, it is proposed that the cloud be called a "microparticle plasma". Since the particle sizes vary over a range of values, the cold plasma many-fluid model is used to analyze the problem, with each fluid representing particles of a particular radius. It is assumed that there is a continuum between one fluid and the next. This model has been used to study plasma waves. Microparticle clouds have been set up in our laboratory in an attempt to realize a "microparticle plasma".